JPH03279812A - Diffractive interferometric encoder - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to an encoder that is capable of highly accurate measurement using light diffraction and interference.

[Conventional technology]

Conventionally, as this type of encoder, for example, JP-A-63-
A device disclosed in Japanese Patent No. 309816 is known, and specifically has a configuration as shown in FIG.

Coherent light emitted from the laser diode 60 is generated into a condensed beam by a condensing lens 61, and is converted to P by a polarizing beam splitter 62.

It is split into two S-polarized beams. Then, each polarized light is reflected by reflecting mirrors 63a and 63b, respectively, and reaches a transmission type diffraction grating 64 with a predetermined equal angle of incidence.

This diffraction grating 64 has a predetermined pitch along the measurement direction, and is provided movably along this measurement direction.

Here, the condensing lens 61 is the polarizing beam splitter 6
21 reflecting mirrors 63a and 63b are condensed onto a diffraction grating 64.

Now, each light beam that has crossed the diffraction grating 64 with a predetermined equal incident angle is diffracted by this diffraction grating 64 in a direction perpendicular to the pitch direction of this diffraction grating 64. The lens 65 converts the light into a parallel beam.

Here, the lens 65 for correcting the change in the diffraction angle is
Even if the diffraction angle changes due to a wavelength fluctuation of the output light due to a temperature change of the laser diode 60, it functions to cancel the influence of this change in the diffraction angle. In other words, even when the diffraction angle changes, the lens 65 is configured so that the two light beams illuminating the diffraction grating 64 in two predetermined directions are made into parallel light beams while being directed in the same direction, and these two-handed light beams always overlap. has been done.

Thereafter, the parallel light flux passing through the lens 65 becomes circularly polarized light by passing through the 174-wavelength plate 66, and is split into two by the beam splitter 67. Then, each of the divided light beams passes through polarizing plates 68a and 68b to a photodetector 69.
The amount of movement of the diffraction grating 64 is changed based on the output signals of the light receiving elements 69a and 69b, which are photoelectrically detected in the measurement direction and correspond to the movement of the diffraction grating 64 in the measurement direction. The direction of movement of the diffraction grating can be detected.

Therefore, in the encoder described above, even when the diffraction angle changes, the interference light can always be detected by the photodetector due to the arrangement of the lens 65 for correcting the change in the diffraction angle, allowing highly accurate measurement. It can be done.

[Problem to be solved by the invention]

In the prior art as described above, the laser diode 6
A change in the diffraction angle of the movable diffraction grating 64 due to a wavelength change due to a temperature change of 0 is corrected by the arrangement of a correction lens 65.

However, adjustments such as focusing and optical axis alignment between the correction lens 65 and the movable diffraction grating 64 are troublesome and difficult.

Further, as described above, although this device is advantageous in changing the diffraction angle due to wavelength fluctuations of the light source, the transmission type movable diffraction grating 64 does not sufficiently compensate for fluctuations in the vertical direction in FIG. be.

That is, due to this variation, the detectors 69a, 69b
The emission direction of the two diffracted lights reaching the top changes, and the condensing state of the detection light via the correction lens 65 changes, so the interference state on the detection surfaces of the detectors 69a and 69b becomes thicker (changes). As a result, the output signal detected by the detector may become unstable.

In addition, in order to obtain two or more sinusoidal output signals with different phase differences and to discriminate the movement direction of the diffraction grating 44, a plurality of optical components such as a polarizing beam splitter 9 wavelength plate and a beam splitter 1 polarizing plate are used. are using.

This not only increases the number of parts, but also complicates the optical configuration, and furthermore, makes adjustment of these components difficult. As a result, there is a risk that not only the size of the device will increase, but also manufacturing costs will increase.

Therefore, the present invention has been made in view of the above problems, and although it has a simple configuration and is extremely easy to adjust in manufacturing, it does not interfere with wavelength fluctuations of the light source due to temperature changes and mechanical fluctuations of the scale. The purpose of the present invention is to provide a low-cost diffraction interference encoder that can perform highly accurate measurements that minimize changes in the state of interference of light.

[Means to solve the problem]

In order to achieve the above object, the present invention specifically provides the second
As shown in Figure A, first and second diffraction gratings (
30, 40), and the first and second diffraction gratings (30, 40)
) for vertically irradiating a coherent parallel beam from the first diffraction grating (30) side;
Due to the diffraction action of the first and second diffraction gratings (30, 40), the first diffraction grating (3o) and the second diffraction grating (
40), in order to cause the diffracted lights crossing each other to interfere with each other at the first position and the second position, which are separated by a distance of
The first and second diffraction gratings (3
0, 40), and each diffraction grating element (3
1a.

31b) for independently photoelectrically detecting only one predetermined diffraction interference light out of the diffraction interference light generated in each diffraction grating element (5a, 5b); 31a, 31b) are arranged at relatively predetermined positions in the pitch direction in order to provide a relative phase difference between the detection signals of the diffraction interference light that are independently detected by the light detection means (5a, 5b). The first and third diffraction gratings (30, 31a, 31b) are provided at a distance of
The amount of relative movement of the second diffraction grating (40) in the pitch direction with respect to the second diffraction grating (40) is detected.

Further, the second diffraction grating (40) is formed of a reflective diffraction grating, and the first and third diffraction gratings (30, 31a,
31b) may be provided integrally on the same plane of the same member.

[For production]

As described above, in the present invention, the first... The second and third diffraction gratings are arranged in parallel at equal intervals, the third diffraction grating is formed to have at least two diffraction grating elements, and a parallel light beam is vertically irradiated from the first diffraction grating side, Due to the diffraction effect of the first to third diffraction gratings, only one predetermined diffraction interference light generated for each diffraction grating element of the third diffraction grating is independently detected by a photodetector.

Specifically, as shown in FIG. 1A, by perpendicularly irradiating the first diffraction grating 30 of scale 3 with a parallel light beam, θ
. In the direction, -nth order light L(-n) is generated, and in the vertical direction, 0th order diffracted light L(0) is generated.

Here, the n-th order light on the right side of the 0th-order light is assumed to be the 10th-order diffracted light, and the one on the left side is the 10th-order diffracted light.

Then, this -〇th order diffracted light L(-n) is reflected at the reflection scale 4.
By illuminating the second diffraction grating 40 at an incident angle θ, n-order diffraction light L(-n, n) is generated in the vertical direction, and zero-order diffraction light L(0) from the first diffraction grating 30 is generated. is vertically incident on the second diffraction grating 40, so that θ. The 10th order light L(0, -n) is generated in the direction. The n-th diffraction light L (n, n) and the 0-th diffraction light L (0
, n) intersect on the third diffraction grating 31a of the scale 3, and these two diffracted lights are diffracted again by the diffraction grating element 31a (hereinafter referred to as the third diffraction grating element 31a) of the third diffraction grating. do. Then, the diffracted lights emitted in the same direction interfere with each other, and as shown in FIG. 1A, for example, interference light (2) is generated in the vertical direction. is guided to a detector (not shown).

Now, the diffraction angle θ is determined by the first diffraction grating 30. For example, when the wavelength of the parallel light beam that is diffracted to the nth order shifts, the diffraction angle becomes θ
. The second diffraction grating 40 and the third diffraction grating element 3 have the same pitch as this first diffraction grating 30, although the pitch changes to □.
Since the diffraction angle of 1a changes to 67H in the same way, the interference light that interferes due to the diffraction effect of this third diffraction grating element 31a is simply horizontally shifted from Io to 11, and the interference state is always kept constant. It will be done.

In addition, as shown in FIG. 1B, even when the second diffraction grating formed on the moving scale fluctuates downward (gap fluctuation) as shown by the dotted line, the diffraction effect of the first to third diffraction gratings can also be obtained. The interference light is simply horizontally shifted from 1 to 2, and the interference state is always kept constant.

As described above, even when the diffraction angle changes due to wavelength fluctuations or when the second diffraction grating etc. fluctuates in the direction perpendicular to the pitch, the diffraction effects of the first to third diffraction gratings cause interference.
Since the two detection lights can be emitted in the same direction, stable interference light with a constant interference state can be obtained.

Furthermore, the present invention provides a structure in which each of the third diffraction grating elements is shifted by a predetermined distance in the pitch direction, thereby giving a predetermined relative phase difference to the output signal of the diffracted interference light that is photoelectrically detected. It is.

This makes it easy to electrically obtain push-pull signals, direction discrimination signals, etc., making it possible to perform highly accurate measurements.

In addition, the present invention can realize a simple configuration in which the number of optical components can be kept to the minimum necessary, which not only facilitates manufacturing adjustments but also makes it possible to reduce the weight and cost of the device. .

〔Example〕

FIG. 2A shows a reflection type encoder in which the diffraction grating 4 is formed of a reflection type diffraction grating.
A first embodiment of the present invention will be described in detail with reference to FIG.

As shown in the figure, a transmission scale 3 and a reflection type movable scale 4 facing and parallel to the transmission scale 3 are provided so as to be movable in the direction of the arrow (horizontal direction).

The transmission scale 3 includes a diffraction grating 3 as a first diffraction grating.
0 and diffraction grating elements 31a and 31b constituting a third diffraction grating are formed on the same plane, and a diffraction grating 40 as a second diffraction grating is formed on the reflective moving scale 4. The diffraction gratings 30.31a, 31b, and 40 provided on each scale are formed at a periodic pitch in which the width of the concave portion and the convex portion are equal.

Above the diffraction grating 30 on the transmission scale 3, a laser diode l and a collimating lens 2 are arranged for supplying a parallel beam of light.

Also, a diffraction grating element 31a on the transmission scale 3.

31b (hereinafter referred to as diffraction gratings 31a and 31b).

), detectors 5a and 5b are arranged above the diffraction gratings 31a and 31b, respectively.

First, a coherent light beam supplied from a light source such as a laser diode 1 is converted into a parallel light beam by a collimating lens 2, and is incident perpendicularly on a diffraction grating 30 formed on a transmission scale 3. .

When this parallel light flux passes through the diffraction grating 30, a refracted light is generated, and in the direction in which this parallel light flux passes, 0-order light L(0) is placed on both sides of the O-order light L(0), θ In the direction, n-th order light L (n) and -n-th order light L (-n) are generated, respectively.

Here, if the wavelength of the light flux supplied from the laser diode 1 is λ1, and the pitch of the diffraction grating element is P1, and the order of the diffracted light generated from the diffraction grating element is n, then the diffraction grating 30
The ±nn-th order diffraction angle θ6 generated by is uniquely given by the following equation.

nλ sin θ. = −(1) However, n is an integer (n: O, ±1.±2 “pa). At this time,
The diffraction angle of the n-th forward beam is θ6゜The diffraction angle of the n-th forward beam is θ-
1, and a relationship of θ6=-0-6 holds true between the two. However, in order to simplify the explanation below, both ±nn-th order diffraction angles will be explained as θi.

Now, consider the 0th-order light L(0), the 1st-order light L(1), and the -1st-order light L(-1) generated from the diffraction grating 30.

As shown in FIG. 2A, while the 0th order light L (0) is perpendicularly incident on the diffraction grating 40 of the reflective moving scale 4,
The order light L(1) and the −1st order light L(−1) obliquely enter the diffraction grating 40 of the reflective moving scale 4 at mutually opposite incident angles θ.

Then, the diffraction grating 40 on the reflective moving scale 4 becomes 0
The first-order light L(0) is re-diffracted in the left and right θ directions.
0,1) and -1st order light L (0, -1), and re-diffracts the first order light L(1) to generate =1st order light L(1,~l) in the vertical direction. . At the same time, the diffraction grating 40 re-diffracts the 1st-order light L (-1) to generate 1st-order light L (-1,l) in the vertical direction.

The diffracted light L(0,1) and diffracted light L(1,-1) generated in this way and the nine diffracted lights L(0,-1) and diffracted light L(-1,l) are respectively They intersect at different positions on the scale 3 separated by the distance (gap) between the transmission scale 3 and the reflective moving scale 4.

Diffraction gratings 31a and 31b are provided corresponding to each of these intersection positions, and these diffraction gratings 31a and 31b are arranged such that, as shown in FIG. They are spaced apart by a predetermined distance so as to produce a shift of 4 pitches.

Then, the diffracted lights that intersect on the pair of diffraction gratings 31a and 31b are respectively diffracted by the diffraction gratings 31a and 31b, and interference lights (lA, IB, IN, ~IN,) is generated.

Here, in this example, the grid arrangement direction (
Two interference lights generated in a direction perpendicular to the pitch direction) (
IA, ■, 3) shall be detected.

This interference light IA is a diffracted light L (0
, -1) and the diffracted light L (-
1, 1), while the interference light IB is
Due to the diffraction action of the diffraction grating 31b, -1st order light of diffracted light L(0°1) is generated in the direction perpendicular to the direction in which the gratings are arranged, and 0th order light of diffracted light L(1, -1) is generated in the transmission direction. It is generated with .

Then, these interference lights (1,, IB) are transmitted to the diffraction grating 3
A pair of detectors 5a arranged corresponding to 1a and 31b,
5b and is detected photoelectrically by the reflective moving scale 4.
Two sinusoidal output signals A and B are obtained depending on the amount of movement.

At this time, the relative phase difference between the two output signals A and H is such that, as described above, the diffraction gratings 31a and 31b are provided so as to produce a relative shift of 1/4 pitch in the moving direction of the second diffraction grating. Therefore, the state is shifted by 90°.

Therefore, it is possible to discriminate the moving direction and detect the moving amount of the reflective moving scale 4 based on the two output signals A and H which are relatively out of phase.

Here, when the movable scale 4 having the second diffraction grating 40 is configured as a reflection type as in this embodiment, the parallel light beam incident on the transmission scale 3 and the interference light to be detected exiting the transmission scale 3 are separated. To separate, use transmission scale 3
It is desirable that the distance g (gap) between the moving scale 4 and the moving scale 4 satisfies the following conditions.

However, n is the order of the n-th destination light for detection diffracted by the first diffraction grating 31 of scale 3.

φ・Parallel beam diameter.

λ: Wavelength of parallel light beam.

P, pitch of the first diffraction grating 31 of scale 3;

Now, as shown in FIG. 2A, a detector (5A) is used.

5B), the diffraction gratings (31a, 31
Interference light between diffracted lights generated in a direction perpendicular to the grating arrangement direction (pitch direction) in b) is detected, but as mentioned above, the θ directions to the left and right of this pitch direction are also shown by dotted lines. Such interference light (IN1゜■9□, Iys,
lN4) occurs.

However, when these interference lights are incident on the detectors 5a and 5b, the output signals A, which are photoelectrically converted by the detectors 5a and 5b, due to diffraction angle fluctuations due to gap fluctuations and wavelength shifts.
There is a possibility that the phase of B will jump significantly, making it difficult to detect the position with high accuracy.

Therefore, in this embodiment, the diffraction grating 31a on the scale 3,
The detectors 5a,
5b is placed apart from the diffraction gratings 31a and 31b. This results in a highly reliable and stable output signal A,
You can get B.

Note that in order to block these interference lights that become noise, a cylindrical light blocking member or the like may be arranged in the aperture 9 having a predetermined aperture shape. This not only makes it possible to reliably extract only the predetermined interference light, but also makes it possible to reduce the distance between the detector and the diffraction gratings 31a and 31b of the transmission scale 3, making it easier to make the apparatus more compact. This can also be applied to the embodiments described below.

According to the above configuration, even if a diffraction angle change and a gap change occur due to wavelength fluctuations of the laser diode 1, two sets of optical fibers proceed along four optical paths and interfere due to the diffraction action of the three diffraction gratings, all of which have the same pitch. Since the optical path lengths of the detection diffracted lights are all the same and the diffraction angles of these diffracted lights are all the same, the interference light to be detected is simply horizontally shifted, and the output photoelectrically detected by each detector increases. The phase of the signal remains constant without changing.

Note that the signal processing may be performed using a method used in a photoelectric encoder. For example, as shown in FIG. 3, a signal processing system that performs signal processing includes a preamplifier 1 that amplifies the output signals A and B obtained from the detectors 5a and 5b.
0, a waveform shaping circuit 11 that shapes each amplified signal into a pulse shape, a direction discrimination circuit 12 that converts each pulse signal into a binary pulse signal, and a pulse signal that has passed through this direction discrimination circuit. Measured values can be obtained by providing a counting circuit 13 and a display section 14 that displays the measurement results obtained by counting pulse signals in a desired format.

Next, FIG. 4A is a diagram showing a second embodiment of the present invention, and members having the same functions as those in FIG. 1 are given the same reference numerals.

The difference from the first embodiment described above is that on the transmission scale 3, third diffraction gratings for obtaining four interference lights are formed in parallel, two on each side with the first diffraction grating 30 in between. Four detectors 5a to 5b are provided corresponding to each of the diffraction gratings 31a to 31b.

FIG. 4B shows a plan view of the transmission scale 3.

When P is the pitch of the grating and m is an integer, as shown in the figure,
The diffraction gratings 31a and 31b are (m+1/4) in the measurement direction.
They are placed at a distance of P.

Further, the diffraction grating 31a and the diffraction grating 31c arranged in parallel thereto, and the diffraction grating 31b and the diffraction grating 31d arranged in parallel thereto are each 1/1/2 in the measurement direction.
In order to shift by 4 pitches, this diffraction grating 31c
and a diffraction grating 31d are arranged at a distance of (m+3/4)P in the measurement direction.

Therefore, the diffraction gratings 31a to 31 on this scale 3
Four interference lights emitted from d in the vertical direction are detected by detectors 5a to 5a.
The phases of the four output signals A-D photoelectrically detected by 5d are shifted by 90 degrees from each other.

Now, if the output signal C of the interference light from the diffraction grating 31c photoelectrically detected by the detector 5c is taken as a reference signal and the phase is θ°, then the relative relationship between this reference signal and the output signal obtained from each detector is The phase difference is 90° for the output signal A of the detector 5A corresponding to the diffraction grating 31a, and 180° for the output signal B of the detector 5B corresponding to the diffraction grating 31b.
The output signal of the detector 5d corresponding to the angle 1d is 270°.

Therefore, if two sets of these four output signals are arbitrarily combined and signal processed, the amount and direction of movement of the reflective moving scale 4 can be detected.

For example, by combining the A and D output signals and the B and C output signals, the amplitude of one of the signals in each set is electrically inverted. That is, detectors 40A and 40D,
Detectors 40B and 40C are electrically connected in antiparallel to 2
generates two push-pull signals. Then, the relative phase difference between these two push-pull signals becomes 90°, and it is preferable to perform position detection and discrimination of the moving direction of the scale based on these two signals.

Thereby, even if the scale 3 and the movable scale 6 fall, the error in reading the detection signal can be minimized.

By adopting a configuration in which detection errors can be electrically compensated in this way, manufacturing tolerances of the diffraction gratings formed on each scale can be reduced.

Next, a third example will be described. FIG. 5 shows the configuration of the third embodiment, and members having the same functions as those in FIG. 1 are given the same reference numerals.

The encoder of this embodiment has a moving scale 4 of a transparent member.
A transmission type diffraction grating 40 is provided on top to make it a transmission type. And each scale 4. 3A.

Each diffraction grating 40.30.31a, 31b on 3B is also
Transmissive regions (or reflective regions) and light-shielding regions are provided in an amplitude grating that is alternately formed at equal pitches.

Then, a transmission type moving scale 4 and a second transmission scale 3
The three scales are arranged in parallel and at equal intervals so that the gap with B is equal to the gap between the first transmission scale 3A and the transmission type moving scale 6.

Here, since this embodiment is basically the same as that of the first embodiment described above, detailed explanation will be omitted.

In the case of this embodiment as well, two pairs of diffraction gratings for generating interference light for detection may be provided as in the second embodiment, and four detectors may be provided correspondingly. good.

By the way, in each of the embodiments of the present invention, as shown in FIG. 2A, for example, the interference lights IA and IP emitted from the third diffraction gratings 31a and 31b in the vertical direction are detected, but the interference lights IA and IP that are emitted from the third diffraction gratings 31a and 31b in the diagonal direction are detected. Since the interference light I Nl+IN4 also has movement information of the moving scale, it is also possible to detect this with a detector.

At this time, the interference state of the interference light I Nl+ INI remains constant even if the diffraction angle and the gap vary due to changes in the wavelength of the light source.

According to this configuration, even when the diameter of the collimating lens 2 becomes large, a compact shape can be ensured.

However, in order to reliably obtain a more stable output signal with high illumination efficiency, it is desirable to detect the interference lights IA and IB vertically emitted from the third diffraction grating, as described in each embodiment.

In addition, in each of the embodiments described above, in order to prevent return light, a light beam composed of, for example, a polarizing beam splitter and a λ/4 plate is placed between the optical path between the collimating lens 2 and the first diffraction grating. An isolator may be placed.

Further, in each of the embodiments of the present invention, the first to third diffraction gratings are unified to be a phase grating or an amplitude grating, but they may be used in combination as appropriate.

Further, according to the present invention, even when the diameter of the light beam illuminating each diffraction grating is made small, a stable output signal of interference light can always be reliably obtained.

Further, in each embodiment of the present invention, the scale having the second diffraction grating is moved, but it may be fixed and the scale having the first and third diffraction gratings moved integrally.

〔Effect of the invention〕

As described above, according to the present invention, despite the simple configuration with an extremely small number of parts, it is possible to eliminate the adverse effects of detection interference light due to wavelength fluctuations of the light source and scale gap fluctuations due to temperature changes, and to improve manufacturing efficiency. Adjustment is also extremely easy, and a high-performance diffraction interference type encoder that can always obtain a stable predetermined output signal can be achieved.

This not only makes it possible to reduce manufacturing costs, but also makes it more compact and lightweight.

In addition, when the second diffraction grating is a reflective type, the illumination system,
Since the detection system and the first and third diffraction gratings can be incorporated into one housing, further cost reduction and compactness of the device can be expected.

Furthermore, as in the first and second embodiments, the second diffraction grating is of a reflective type and the first diffraction grating and third diffraction grating are integrally structured, so that the number of parts can be extremely reduced. Since the number of adjustment points during manufacturing can be significantly reduced, significant cost reductions can be expected.

[Brief explanation of drawings]

FIG. 1A is a diagram showing how the light source wavelength changes in the encoder of the present invention. FIG. 1B is a diagram showing a situation when the gap varies in the encoder of the present invention. FIG. 2A is a diagram showing a schematic configuration of a first embodiment according to the present invention. FIG. 2B is a plan view of the transmission scale 3 of FIG. 2A. FIG. 3 is a block diagram showing the signal processing system of the first embodiment. FIG. 4A is a perspective view showing the structure of a second embodiment of the present invention. FIG. 4B is a plan view of the transmission scale 3 of FIG. 4A. FIG. 5 is a diagram showing a schematic configuration of a third embodiment according to the present invention. FIG. 6 is a schematic configuration diagram of a conventional encoder. [Explanation of symbols of main parts] 3, 3A, 3B, 4...--Scale 31...First diffraction grating 40--°...-°Second diffraction grating

Claims (1)

[Claims] 1) first and second diffraction gratings arranged in parallel and having equal periodic pitches;
a collimated beam supply means for perpendicularly irradiating a coherent collimated beam onto the diffraction grating from the first diffraction grating side; and a distance between the first diffraction grating and the second diffraction grating by the diffraction action of the first and second diffraction gratings. In order to cause the diffracted lights crossing each other at the first position and the second position, which are separated by a third diffraction grating having diffraction grating elements formed at equal periodic pitches in parallel and in the same direction as the first and second diffraction gratings; and diffraction interference light generated for each diffraction grating element of the third diffraction grating. and a photodetecting means for independently photoelectrically detecting only one predetermined diffracted interference light among the third diffraction gratings, each of the diffraction grating elements of the third diffraction grating has a photodetection means for independently photoelectrically detecting only one predetermined diffraction interference light among the third diffraction gratings, and each of the diffraction grating elements of the third diffraction grating has a In order to provide a relative phase difference between the detection signals of the diffraction interference light, the second diffraction grating is provided at a relatively predetermined distance in the pitch direction, and A diffraction interference encoder characterized by detecting the amount of relative movement in the pitch direction. 2) the second diffraction grating is formed of a reflective diffraction grating;
2. The diffraction interference encoder according to claim 1, wherein the first and third diffraction gratings are provided on the same plane.